The present invention relates to a series of new polycyclic aromatic alkaloids having a pyrido[2,3,4-k,l]acridine skeleton which have cytotoxic properties and which can therefore be used in the treatment of malignant tumours. The invention also provides methods and compositions using these new compounds as well as processes for their preparation.
The polycyclic aromatic alkaloids based on the pyrido[2,3,4-k,l]acridine skeleton are a growing class of ascidian metabolites that often exhibit a variety of interesting biological properties, including antitumour activity.1 This class of compounds comprises three main structural types, depending of the position of the fusion between the parent structure and additional rings present in the natural product. For example, the cystoditines2 have the base skeleton mentioned above, while amphimedine,3 meridine4 and cystodamine5 bear an additional pyridine ring attached to the h bond; ascididemin,6 its derivatives,7 the kuanoniamines8 and shermilamine A9 show this additional ring at the i face, and eilatine at both.10 Our target compounds can be regarded as regioisomers both of meridine and amphimedine, but they have not been so far isolated from natural sources.
Most of these prior art compounds exhibit very interesting cytotoxic properties towards a range of tumor cell lines. Although their mechanism of action is not clearly established, three general observations emerge from published biological data:11 a) they are intercalating agents; b) they disrupt DNA and RNA synthesis, with little effect on protein synthesis; c) they inhibit topoisomerase II, which is the mechanism normally accepted for their antitumour activity.
Biological studies on pyridoacridines are severely limited due to their very low availability from natural sources, and therefore the study of their mechanism of action and the establishment of reliable structure-activity relationships requires the development of efficient synthetic routes.
We have now discovered a new family of polycyclic aromatic alkaloids having a pyrido[2,3,4-k,l]lacridine skeleton which show excellent antitumour activity.
Thus, in a first aspect of the present invention there is provided compounds having the general formula (I): 
wherein:
X is selected from the group consisting of O, and NR3, where R3 represents a lower alkyl group;
Y is selected from the group consisting of CH and N;
R1 and R2 are independently selected from the group consisting of NH2, NHR4 and NR52, where R4 and R5 each represent a lower alkyl group, or R1 and R2 together represent a cycle selected from (a), (b) and (c): 
wherein R6, R7 and R8 are independently selected from the group consisting of hydrogen atoms, lower alkyl groups, hydroxy groups and lower alkoxy groups; and
Z is selected from the group consisting of O and NH; and pharmaceutically acceptable salts thereof.
In the definitions of the groups in formula (I), the lower alkyl groups and the lower alkyl moiety of the lower alkoxy groups are straight-chain or branched alkyl groups having 1 to 6 carbon atoms such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, neopentyl and hexyl groups.
For those compounds of the present invention wherein R1 and R2 together represent a cycle of formula (a), R6 and R7 are preferably independently selected from the group consisting of hydrogen atoms, alkyl groups having from 1 to 4 carbon atoms, hydroxy groups and alkoxy groups having from 1 to 4 carbon atoms; more preferably, R6 and R7 are independently selected from the group consisting of hydrogen atoms, methyl groups and ethyl groups; and, most preferably, R6 represents a methyl group and R7 represents a hydrogen atom. Furthermore, where R1 and R2 together represent a cycle of formula (a), it is preferable that Z represents a group of formula NH.
For those compounds of the present invention wherein R1 and R2 together represent a cycle of formula (b) or (c), R6, R7 and R8 are preferably selected from the group consisting of hydrogen atoms, alkyl groups having from 1 to 4 carbon atoms, hydroxy groups and alkoxy groups having from 1 to 4 carbon atoms; more preferably, R6, R7 and R8 are independently selected from the group consisting of hydrogen atoms, methyl groups, ethyl groups and hydroxy groups; and, most preferably, R6 represents a hydroxy group and R7 and R8 each represent a hydrogen atom.
In a preferred embodiment, the present invention relates to novel synthetic compounds of general structure (II) or (III): 
wherein R6, R7 and R8 are as defined above.
Of the compounds of general formula (II), we particularly prefer the compound wherein R6 represents a methyl group and R7 represents a hydrogen atom (Compound No. IB-96213).
Of the compounds of general formula (III), we particularly prefer the compound wherein R7 and R8 both represent a hydrogen atom (Compound No. IB-98205).
The present invention also provides a method for treating a mammal affected by a malignant tumor sensitive to a compound having the general formula (I), which comprises administering to the affected individual a therapeutically effective amount of the compound having the general formula (I) or a pharmaceutical composition thereof.
The present invention further provides pharmaceutical compositions, particularly useful in the treatment of malignant tumors, which contain as the active ingredient a compound having the general formula (I), as well as a process for the preparation of said compositions.
A further aspect of the present invention provides a method for preparing the compounds of general formula (I) and, in particular, Compounds Nos. IB-96213 and IB-98205.
Our strategy for the synthesis of the target compounds involves an hetero Diels-Alder reaction between an o-nitrogenated 4-aryl-1-dimethylamino-1,4-azadiene and a suitable heterocyclic quinone, followed by nucleophilic cyclization onto one of the quinone carbonyls (Scheme 1). 
Among the several 4-aryl-l-azadienes assayed, the best results were obtained for the o-nitro and the o-(trifluoroacetamido) derivatives. The first of them (compound 1a) was prepared from commercially available o-nitrocinnamaldehyde using a known procedure12. For the second azadiene (compound 1b), two alternative syntheses were devised (Scheme 2). The first of them involved trifluoroacetylation of the corresponding o-amino derivative 2, prepared by reduction of compound 1a12. In the second synthesis, o-aminobenzaldehyde (3) was N-trifluoroacetylated and the amide was used as a Wadsworth-Emmons coupling partner with compound 5.13 
The reaction between 1b and quinones 614 to produce compounds of formula (II) was carried out in refluxing chloroform, and led to a mixture of the Diels-Alder adducts 7 and the secondary products 8. Compounds 7 were aromatized to 9 by refluxing in the presence of Pd-C in methanol solution, and 9 was finally transformed into the desired pentacyclic derivatives 10 [general structure (II)] by hydrolytic cyclization (Scheme 3). 
The use of 6-bromo-4-chloroquinolinequinones 11 as dienophiles led to reversal of the regioselectivity of the hetero Diels-Alder reaction. Thus, treatment of azadiene 1a with 11 under ultrasound irradiation afforded compound 12. Hydrogenation of 12 in the presence of Pd-C followed by workup with trifluoroacetic acid gave pyridoacridines 13 [general structure (III)] (Scheme 4). 
As examples, the detailed procedure for the synthesis of 12-methyl-9H-benzo[b]pyrido[4,3,2-d,e](1,7)phenantr IB-96213 (general structure 10, where R6=CH3, R7=H) and 9-hydroxybenzo[b]pyrido[4,3,2-d,e](1,10)phenantrolin-8-one IB-98205 (general structure 13, where R7=R8=H) are given in the experimental section.
The compounds of the present invention are cytotoxic, compounds such as IB-96213 and IB-98205 exhibiting antitumor activity, especially against cell lines derived from human solid tumors, such as human lung carcinoma, human colon carcinoma and human melanoma, and, the like. They are also active against other tumor cell lines, like leukemia and lymphoma. Compounds of formula (I), such as IB-96213 and IB-98205, have in vitro antitumor selectivity for solid tumors.
Examples of pharmaceutical compositions include any solid (tablets, pills, capsules, granules, etc.) or liquid (solutions, suspensions or emulsions) dosage form, with suitable formulation of oral, topical or parenteral administration, and they may contain the pure compound or in combination with any carrier or other pharmacologically active compounds. These compositions may need to be sterile when administered parenterally.
The correct dosage of a pharmaceutical composition comprising compounds with formula (I), will vary according to the pharmaceutical formulation, the mode of application, and the particular situs, host and tumor being treated. Other factors like age, body weight, sex, diet, time of administration, rate of excretion, condition of the host, drug combinations, reaction sensitivities and severity of the disease shall be taken into account. Administration can be carried out continuously or periodically within the maximum tolerated dose.
Experimental
The invention is further illustrated by the following Examples, which demonstrate the preparation of various of the compounds of the present invention. The reagents used were of commercial origin (Aldrich, Fluka) and were employed without further purification. Solvents (SDS, Scharlau) were purified and dried by standard procedures. Reactions were monitored by thin-layer chromatography, using Scharlau and Macherey-Nagel plates with fluorescent indicator. Separations by flash liquid chromatography were performed using silica gel SDS 60 ACC (230-400 mesh).
Melting points are uncorrected, and were determined in open capillary tubes, using a Bxc3xcchi immersion apparatus or a Hoffler hot stage microscope. Combustion elemental analyses were obtained by the Servicio de Microanalisis Elemental, Universidad Complutense, using a Perkin Elmer 2400 CHN analyzer. Spectroscopic data were obtained with the following instruments: IR, Perkin Elmer 577 and Perki Elmer Paragon 1000 FT-IR; NMR, Varian VXR-300 (300 MHz for 1H and 75 MHz for 13C) and Bruker AC-250 (250 MHz for 1H and 63 MHz for 13C).